In recent years, a number of ultra super critical boilers which are operated at an increased temperature and pressure to achieve high efficiency are newly constructed in the world.
Specifically, in some projects, the steam temperature, which has so far been about 600° C., is further increased to 650° C. or higher and further to 700° C. or higher. This is based on the fact that energy saving, effective use of resources, and reduction in CO2 gas emission for environmental preservation are challenges to solve energy problems, and are included in important industrial policies. In the case of power generating boilers burning a fossil fuel and reactors for the chemical industry, a highly efficient ultra super critical boilers and reactors are advantageous.
Such high temperature and pressure of steam also increases the temperature of a superheater tube of boiler, a reactor tube for the chemical industry, and a thick plate and a forging used as a part having heat resistance and pressure resistance to 700° C. or higher at the time of actual operation. Therefore, an alloy used in a harsh environment for a long period of time must be of excellent in not only high-temperature strength and high-temperature corrosion resistance but also long-term stability of metal micro-structure, creep rupture ductility, and creep fatigue resistance.
Further, during the maintenance work such as repair after long-term use, a material aged in a long period of time needs to be cut, worked, or welded, and therefore, not only the characteristics for a new material but also the soundness of an aged material have been required strongly in recent years.
In meeting the severe requirements, an Fe-based alloy such as an austenitic stainless steel suffers lack of creep rupture strength. Therefore, it is inevitable to use a Ni-based alloy in which the precipitation of a γ′ phase or the like is utilized.
Accordingly, Patent Documents 1 to 8 disclose Ni-based alloys that contain Mo and/or W to achieve solid-solution strengthening, and contain Al and Ti to utilize precipitation strengthening of the γ′ phase, which is an intermetallic compound, or specifically utilize precipitation strengthening of Ni3(Al,Ti) for use in the above-described harsh high-temperature environment.
In the alloys disclosed in Patent Documents 4 to 6, since 28% or more of Cr is contained, an a large amount of α-Cr phase having a bcc structure also precipitates and contributes to strengthening.